It is known for circuit breakers to be connected in series, wherein the circuit breaker which is arranged immediately downstream from the current feed is generally followed on the load side by a plurality of circuit breakers. It is also known for circuit breakers to be tripped selectively in the event of a short. In this case, selectively means that that circuit breaker is in each case tripped which is located closest to the location of the short, seen from the current feed, as a result of which no more loads than those which are unavoidable are affected by the disconnection. In order to achieve this, the mutual upstream and downstream circuit breakers can be the subject of current and time grading, in which case the circuit breaker close to the feed has longer disconnection times than those which are further away from the feed. This means that the upstream circuit breakers are disconnected only when they themselves are closest to the short, or else in the event of a protection failure of a downstream circuit breaker, where a downstream circuit breaker does not trip even though it should trip. In the case of current and time grading such as this, circuit breakers are configured such that they trip at ever greater currents and after ever longer times in the feed direction, in the event of a short.
In order to shorten the relatively long disconnection times that this requires, the downstream circuit breakers can in each case signal to the upstream circuit breakers that they are carrying out the disconnection. However, this requires a relatively large amount of additional circuit complexity.
In order to reduce this complexity, WO 2006/108860 A1proposes that an upstream circuit breaker monitors one or more downstream circuit breakers, with the monitoring being carried out on the basis of the output-side impedance. The impedance is significant in the event of a short, when one of the downstream circuit breakers should in this case trip, that is to say it already opens its switching contacts in order to interrupt the output-side circuit, and this is associated with the formation of a switching arc between the switching contacts. This switching arc results in a typical rise in the output-side impedance, which is detected by the upstream circuit breaker which can identify that a downstream circuit breaker has already tripped, and that it need not trip itself. If the upstream circuit breaker has already started the process of opening its own switching contacts, then it can close them again.